Polyphenylene sulfide ketones (hereinafter abbreviated as PPSK) having a recurring unit represented by the general formula ##STR2## have such high heat resistance (glass transition point, 137.degree. C.; crystal melting point, 352.degree. C.) that they are anticipated to find utility in a broad range of applications including the electric, electronics and automotive industries.
Conventionally, PPSK has been synthesized from dihaloaromatic compounds and alkali metal sulfides in polar organic solvents and two processes based on this approach are described in Indian Journal of Chemistry, 21, 501 (1982) and JP-A-60-58435 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In the process described in the former reference, the polymerization temperature is as low as 170.degree. to 180.degree. C. and the reaction is performed by one-stage heating. Thus, the resulting polymer has only a low solution viscosity of 0.26 (H.sub.2 SO.sub.4, 0.5 g/dl, 28.degree. C.). In the process described in JP-A-60-38435, the alkali metal sulfide is deprived of free water or the water of hydration by heating in a polar organic solvent and the reaction mixture is further polymerized by two-stage heating. In spite of this refined approach, the resulting polymer has low melting point and the part formed from it is very brittle.
U.S. Pat. No. 4,716,212 describes a process in which PPSK of high molecular weight is produced by adding sodium hydroxide and a slight excess of sodium hydrosulfide to the system. According to the Examples given therein, however, a heat treatment conducted at 316.degree. C. for 120 minutes in an air atmosphere rendered the reaction product insoluble in concentrated sulfuric acid. This phenomenon suggests the occurrence of a marked structural change due to a crosslinking reaction, which led to low thermal stability of the polymer.
Because of its crystalline nature and high melting point, PPSK needs a high processing temperature and whether it is thermally stable or not presents an important problem in its shaping. However, no review has been made on the thermal stability of PPSK.
To fill this gap, the present inventors have made extensive and intensive studies and found that PPSK having better thermal stability than those produced by known processes could be obtained by improving the conditions of polymerization in the following way: an alkali metal sulfide used as a reactant is not deprived of free water or the water of hydration by heating, and reaction is performed at low temperatures in the initial period of polymerization until the percentage of residual unreacted dihalogen compound is reduced to 2% or below. The present invention has been accomplished on the basis of this finding.